Re: [PATCH v7 5/6] xen/riscv: introduce an implementation of macros from <asm/bug.h>

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.08.2023 14:05, Oleksii Kurochko wrote:
> --- a/xen/arch/riscv/traps.c
> +++ b/xen/arch/riscv/traps.c
> @@ -5,6 +5,8 @@
>   * RISC-V Trap handlers
>   */
>  
> +#include <xen/bug.h>
> +#include <xen/errno.h>
>  #include <xen/lib.h>
>  
>  #include <asm/csr.h>
> @@ -12,6 +14,8 @@
>  #include <asm/processor.h>
>  #include <asm/traps.h>
>  
> +#define cast_to_bug_frame(addr) ((const struct bug_frame *)(addr))
> +
>  /*
>   * Initialize the trap handling.
>   *
> @@ -101,7 +105,131 @@ static void do_unexpected_trap(const struct 
> cpu_user_regs *regs)
>      die();
>  }
>  
> +void show_execution_state(const struct cpu_user_regs *regs)
> +{
> +    printk("implement show_execution_state(regs)\n");
> +}
> +
> +/*
> + * TODO: generic do_bug_frame() should be used instead of current
> + * implementation panic(), printk() and find_text_region()
> + * (virtual memory?) will be ready/merged
> + */
> +int do_bug_frame(const struct cpu_user_regs *regs, vaddr_t pc)
> +{
> +    const struct bug_frame *start, *end;
> +    const struct bug_frame *bug = NULL;
> +    unsigned int id = 0;

Pointless initializer.

> +    const char *filename, *predicate;
> +    int lineno;
> +
> +    static const struct bug_frame *bug_frames[] = {

You likely want another const here.

> +        &__start_bug_frames[0],
> +        &__stop_bug_frames_0[0],
> +        &__stop_bug_frames_1[0],
> +        &__stop_bug_frames_2[0],
> +        &__stop_bug_frames_3[0],
> +    };
> +
> +    for ( id = 0; id < BUGFRAME_NR; id++ )
> +    {
> +        start = cast_to_bug_frame(bug_frames[id]);
> +        end   = cast_to_bug_frame(bug_frames[id + 1]);

Why these casts (and then even hidden in a macro)? The array elements
look to already be of appropriate type.

> +        while ( start != end )
> +        {
> +            if ( (vaddr_t)bug_loc(start) == pc )
> +            {
> +                bug = start;
> +                goto found;
> +            }
> +
> +            start++;
> +        }
> +    }
> +
> + found:
> +    if ( bug == NULL )
> +        return -ENOENT;
> +
> +    if ( id == BUGFRAME_run_fn )
> +    {
> +        void (*fn)(const struct cpu_user_regs *) = bug_ptr(bug);
> +
> +        fn(regs);
> +
> +        goto end;
> +    }
> +
> +    /* WARN, BUG or ASSERT: decode the filename pointer and line number. */
> +    filename = bug_ptr(bug);
> +    lineno = bug_line(bug);
> +
> +    switch ( id )
> +    {
> +    case BUGFRAME_warn:
> +        printk("Xen WARN at %s:%d\n", filename, lineno);
> +
> +        show_execution_state(regs);
> +
> +        goto end;
> +
> +    case BUGFRAME_bug:
> +        printk("Xen BUG at %s:%d\n", filename, lineno);
> +
> +        show_execution_state(regs);
> +
> +        printk("change wait_for_interrupt to panic() when common is 
> available\n");
> +        die();
> +
> +    case BUGFRAME_assert:
> +        /* ASSERT: decode the predicate string pointer. */
> +        predicate = bug_msg(bug);
> +
> +        printk("Assertion %s failed at %s:%d\n", predicate, filename, 
> lineno);
> +
> +        show_execution_state(regs);
> +
> +        printk("change wait_for_interrupt to panic() when common is 
> available\n");
> +        die();
> +    }
> +
> +    return -EINVAL;
> +
> + end:
> +    return 0;
> +}
> +
> +static bool is_valid_bugaddr(uint32_t insn)
> +{
> +    return insn == BUG_INSN_32 ||
> +           (insn & COMPRESSED_INSN_MASK) == BUG_INSN_16;
> +}

Why "addr" in the name when this takes an insn as argument?

> +/* Should be used only in Xen code ? */

What is this question about? With ...

> +static uint32_t read_instr(unsigned long pc)
> +{
> +    uint16_t instr16 = *(uint16_t *)pc;
> +
> +    if ( GET_INSN_LENGTH(instr16) == 2 )
> +        return (uint32_t)instr16;

(I don't think this cast is needed.)

> +    else
> +        return *(uint32_t *)pc;
> +}

... there still being a double read here, do you perhaps mean to
make a statement (that this code isn't safe to use on guest code)?

>  void do_trap(struct cpu_user_regs *cpu_regs)
>  {
> +    register_t pc = cpu_regs->sepc;
> +    uint32_t instr = read_instr(pc);
> +
> +    if ( is_valid_bugaddr(instr) )
> +    {
> +        if ( !do_bug_frame(cpu_regs, pc) )
> +        {
> +            cpu_regs->sepc += GET_INSN_LENGTH(instr);
> +            return;
> +        }
> +    }
> +
>      do_unexpected_trap(cpu_regs);
>  }